Radio signaling system



T E Q T w T 1% 4 0 I INVENTOR L. 7i H/NTON BI ATTORNEY May 2, 1933.

Patented May 2, 1933 UNE'I'ED STATES PATENT OFFICE LESLIE TURNER HINTON,OF PAR-IS, FRANCE, ASSIGNOR TO WESTERN ELECTRIC COM- PANY, INCORPORATED,OF NEW YORK, N. Y., A CORPORATION OF NEW YORK RADIO SIGNALING SYSTEMThis invention relates to signaling systems and particularly to suchsystems wherein intelligencewhich may be in the form of s1 eech, music,pictures, etc, is transmitted by means of modulated carrier waves, thelatter being subsequently demodulated to reproduce the original signals.

In such a high frequency signaling system a certain quality degradationof the transmitted signal occurs and one of the objects of the presentinvention is to reduce this quality degradation and to improve thestandard of quality of the transmitted signals.

A substantial portion of this quality degradation results from theproduction in such signaling systems of spurious harmonic fre quencies,some of them falling within the signal frequency band and thus causingdistortion.

The analysis of the production of such spurious harmonics is somewhatcomplicated but in order to render the following more clear a fewexplanations will be given.

In a high frequency signaling system the received signal usuallyconsists of a carrier wave and of two side bands. It is a known factthat particularly in short wave operation a selective fading effectdevelops. This effect is usually explained by the phase dif ferences inthe received signals when they are received over different spacechannels. The phase differences vary with the weather, the

, time of the day, and with other conditions,

and produce a marked change in the attenuation of relatively narrowhands over the whole width of the band received. 'For example, assumingthat there is in transmission a carrier frequency and side bands eachhaving 3 kilocycles band width, from time to time narrow bands withinthe total of 6 kiloeycles will have their attenuation considerablyincreased. It has been found that a narrow band containing the carrierfrequency might fade while the frequency in the side bands say, 1100cycles from the carrier hand on each side, will not experience the sameattenuation.

Considering now what happens in the receiving circuit, it will be seenthat if a single frequency of 1100 cycles is transmitted, the

501,825, and in Great Britain January 2, 1930.

receiver picks up three frequencies, viZ., 0+ 1100, C and C 1100, whereC is the carrier frequency. Usually the amplitude of the carrier wave isconsiderably greater than that of the side bands. The amplitude of the1100 cycles received is afunction of the amplitude of the carrier waveand of the side bands whereas that of the unwanted modulation productsof 2200 cycles is a function of the amplitude of the side band alone.

Under the selective fading conditions mentioned above, the amplitude ofthe carrier wave may decrease considerably while the amplitude of theside bands may remain unchanged. Then the ratio of the received signalsof 1100 cycles to the one of 2200 cycles will be considerably changed,that of 2200 cycles appearing in such an amount that it harms thequality of the transmit-ted signals.

It has been noted that this effect is more noticeable in telephony thanin telegraphy, probably due to the production of new and unintelligiblefrequencies. In the case of a high frequency signaling system usingsignal frequency band inversion it should be noted that the spuriousharmonics produced may have a distorting eifect different from the onethey have in a system which does not use signal frequency bandinversion. In some cases the spurious frequencies in question are moreobjectionable to the ear, in a system using frequency band inversionthan in a system dispensing with frequency band inversion. This efiectmay be explained as follows in the case of telephony.

In a high frequency signaling system, harmonics of the originalfrequencies are produced whenever a modulated wave is demodulated in adevice operating by means of a nonlinear characteristic. For example,these harmonics are always produced in the usual types of receiversemploying an ordinary detector. Usually the predominant harmonic is thesecond (i. e. 2f where f is the fundamental of the si al frequency) andthe ratio of the ampiitn" e of the second hannonic to the amplitude ofthe fundamental is a function of the percentage modulation.

With ordinary systems of transmission, however, the degradation ofquality due to hand and if the filters used have very sharp theinteraction of these harn'ionics is not parti cularly serious, thefrequencies produced are definitely in harmonic relation to the signalfrequency and the ear is able to accept them, the net result being aslight (lOCIOLlEK. in the apparent shrillness or hardness of tone in thecase of telephony. lVhen inverted speech is transmitted on the otherhand, the harl'nonics produced. in the receiving detector are lnn-iuwm'of the inverted frequencies and consequently will have no simplerelation to the signal frequencies after the Whole band has beenre-invertcd. The ear is therefore unable to accept such spurious frequencies and per-(hives them as objectionable and interfering noises.

It should be noted that in any signaling system utilizing a modulatedcarrier wave (i. e. using inverted or non-inverted signal frequencybands) other spurious frequencies which are not harmonics of theoriginal frequencies are introduced. This occurs whenever, as in thecase of music broadcasting, more than one frequency is transmitted at atime. In such case the various signal frequencies beat together, givingterms whose frequencies are equal to the sums and differences of all theoriginal signal. frequencies taken together in pairs. This a degradationof quality which. is often noticeable in the reception of ordinarybroadcasting.

According to a feature of the present invention, means are provided forminimizing the influence of spurious harmonics in a high frequencysignaling system.

According to another feature of the present invention the spuriousharmonics are reduced by suitably choosing the relative positions in thefrequency spectrum of the carrier frequency'and of the signal frequencyhand.

If the spacing frequency, that is to say, the distance in the frequencyspectrum between the carrier frequency and the lower frequency of thesignal frequency band, is different from zero, some parasiticintermodulation products falling within the signal frequency band arecut out.

If the spacing frequency is equal to the frequency band Width of thesignal frequency cut-off frequencies, substantially all these spuriousor parasitic inter-modulation prodnets are cut out.

One advantage of the present invention is that the present method forreducing the influence of spurious harmonies may be applied eitherto asecret signaling system wherein the whole frequency band is inverted, orto a system wherein only some sub-bands of the signal freqnency bandsare inverted. In either of the cases an improvement in the quality ofthe transmitted signals is obtained.

It should be emphasized that although the present system is particularlysuitable for a fsecret signaling system using signal frequency bandinversion, the method of choosing the relative position in the frequencyspectrum of the carrier fre 1;lency and of the sigz'ial frequency band,may he used in any signaling svstmn :2 nd will improve the quality oftransmission.

According to the invention a high frequency signaling system is providedwith the side bands located at such distance from the carrier band thatundesirable frequencies are substantially eliminated.

The invention will be described. in connection with the accon'rnanyingdrawing which shows, in y 'wa y of example, some arrangements forcarrying the invention into effect.

Referring to the drawing, Figs. 1 and 2 il lustrate the position in thefrequency spectrum of the side band and of the carrier frequency.

Fig. 3 illustrates the position of the side hands when they have beenspaced from the carrier frequency.

Figs. 4c and 5 show improved high frequency signaling systems.

l-V hen transmitting a signal frequency band of width a, spuriousfrequencies are produced in the received signals occupying a hand offrequency of width 2a. As shown in Fig. 1, the original signal frequencyband occupies the position 1 in the frequency spectrum whereas 2represents the range covered by the spurious frequencies which areobtained during selective fading conditions.

The relation of the carrier frequency 3 with regard to the side bands 4tand l is also shown in Fig. 1 and it should be noted that the side bands1- and 9 occupy a position in the frequency spectrum immediatelyadjacent to that of the carrier frequency.

It may be shown, as illustrated in Fig. 2, that if the side bands areseparated from the carrier wave in the received signals so that if thewidth of the side band is (1 cycles, then the lowest speech frequency inthe side hand must be at a cycles from the carrier Wave G and under thiscondition the useful received signal will be between a and 2a cycles,While the undesirable frequencies will be between 2a and 4a cycles.

Therefore the useful signals can be separated from the useless ones bymeans of filters or other known devices producing a similar efl'ect.

In order to determine the separation of a number of side bands from thefrequency of a carrier wave, it may be assumed that the width of theside bands nearest to the carrier is a cycles as shown in Fig. 3. Thereis a separation equal to a cycles of those next adjacent 5 cycles nextto the carrier frequency and consequently alternate bands, one usefuland the other useless, are produced and will follow the law 6;, 2a, 6,2b. In Fig. 3, C is a line of symmetry; the complete drawing to the leftof G is not shown,

being identical with that shown to the right flay,

The width of the useful baml 0 should not be greater than that of theband a.

Fig. 4; shows the system schrnnatical iy in the form applied to a simpletckqihouc systmn. A band having a frequency range from 0 to 3 kilocyclesis increased in frequency by known means such as by a modulator MOD andselection of a side band by means of the filter F, and demodulators ifnecessary and is fed to the transmitter T as a frequency band from 3 to(i hilocycles. Here it is combined with the carrier frequency by any ofthe known methods and it is transmitted to the receiving point. which isprovided with a detector 1 the useful received signal will be comprisedbetween 3 to 0 kilocycles and the useless one between 6 to 12lrilocycles. The two bands are then separated by a filter F which allowsthe passage of the useful signal alone. By means of a frequencystep-down device which may comprise a filter F and a frequency changingdevice FG (which may be similar to the stepup arrangement used at thetransmitting end) the frequency band is brought to the position of 0 to3 kilocycles, thus giving a frequency band at the receiving point havingthe same position in the frequency spectrum the ori inal frequency bandat the sending point.

*ig. 5 shows an alternative way for modifying the position of a signalfrequency band in the frequency spectrum so as to reduce spuriousfrequencies.

Referring to Fig. 5 the speech signals are applied to an apparatus T fortransforming sound vibrations into electrical vibrations. The saidelectrical vibrations are applied to a low pass filter LP having acut-off frequency of 3000 cycles per second. The filtered signalsthereafter are applied to a modulator MODl, supplied with a modulatingfrequency of say 20 kilocycles, and the upper side band thus produced isselected by means of a filter F having a suitable pass range, forinstance a pass range between 20 and 23 kilocycles. This side hand thenenters a demodulator MOD2 supplied with a carrier frequency of say 26kilocycles the lower side band being then selected by means of a lowpass filter LP having a cut-off frequency of 6000 cycles. In the outputof this filter there is obtained an inverted speech band with afrequency range of 3000 to 6000 cycles. This is applied to thetransmitter T and sent to the receiving station R. In the receivinginverter, the inverted speech is applied to a high pass filter HP havinga cut-ofi frequency of 3000 cycles and is thereafter applied to amodulator MOD.1 supplied with a carrier frequency of 20 kilocycles, theupper side band being selected by means of a band pass filter BF ofsuitable range, for example a. pass filter of 23 to 26 kilocycles. Theout At the receivin .ioint' put of filter BF is applied to a demodulatorMOD2 which is supplied with a carrier frequency of 26 lrilocycles, thelower side band being selected by means of a low pass filter Ll" with acut-oil frequency of 3000 cycles, normal speech being ()btt-tll'md afterthis operation. The normal. speech vibrations are applied to a device Tadapted to transform electrical vibrations into sound waves.

The beat notes between the side hands, when the carrier frequency fadesout, cause, as it has been pointed out above, spurious harmonicfrequencies and particularly a strong second harmonic, but this spuriousfrequency is now outside the gnul frequency range in the output of thereceiving inverter.

The present systems permit an increase in the degree of modulation atthe transmitter since the spurious harmonic frequencies are no longertroublesome. In certain cases the present system. allows a reduction ofthe signal to noise ratio, that is to say a reduction of the ratio ofthe received field strength to the field strength due to static inasmuchas this reduction of signd strength is offset by the increased perccnmodulation that may be used.

The principle of suitably selecting the positionin the frequencyspectrum of the carrier frequency and of the signal frequency band forreducing disturbing intermodulation products and for providing thefacilities above mentioned is of considerable importance and the broadidea underlying this invention may easily be applied to other schemes ofmodulation such as frequency or phase modulation systems.

What is claimed is:

1. The method of signaling which comprises generating a pair ofsidebands displaced by a frequency interval from the neighborhood of anassociated carrier Wave, transmitting said carrier and displacedsidebands, detecting displaced signals therefrom with incidentalproduction of combination frequencies due to inter-modulation betweenthe two sidebands, said combination frequencies lying mainly outside thefrequency range of said signals due to said displacement of thesidebands, selecting said signals and translating them to their originalfrequency range substantially free from said combinat'ion frequencies.

2. The method of transmission of signals employing difierent bands ofmodulated waves, characterized in this, that the frequencies of theunmodulated waves relative to frequencies of said signals, are soselected that the side-bands generated by modulation of said waves bysaid signals are separated in the frequency spectrum by at least twicethe Width of a single side-band.

3. A method in accordance with claim 2, characterized in this, thatunmodulated waves of two different frequencies are supplied, the Wavesresulting from modulation of the Wave of one frequency by signals beingfreed of any component of the unmodulated wave frequency, and then usedto modu- 5 late the Wave of said other frequency.

4:. A method in accordance With claim 2, characterized in this, thatunmodulated Waves of two different frequencies are supplied, the Wavesgenerated by modulation of one of said frequencies by signals beingfiltered to suppress all components except the lower side-bands, andthen used to modulate the wave of said other frequency.

5. A high frequency signaling system using signal frequency inversion,wherein the transmitting wave comprises a carrier Wave and side-hands,said side-bands inversely related in frequencies to the signalfrequencies, and spaced from said carrier and each other in thefrequency spectrum by at least twice the Width of one of saidside-bands.

6. In a secrecy signaling system, employing signal. frequency bandinversion, the method of substantially eliminating effects in thereceived and translated Waves of the quality degrading combination andintermodulation frequencies produced in transmission, which comprisesspacing the inverted signal frequency bands, in the transmitted Waves,at least twice the signal frequency range apart, and separating saidquality degrading frequencies from the useful components of the receivedWaves by frequency e discriminating means.

In Witness whereof, I hereunto subscribe my name this 18th day ofNovember, 1980.

LESLIE TURNER- HINTON.

